Endothelium 1977: a review

Integrin alphavbeta5 regulates lung vascular permeability and pulmonary endothelial barrier function

Increased lung vascular permeability is an important contributor to respiratory failure in acute lung injury (ALI). We found that a function-blocking antibody against the integrin alphavbeta5 prevented development of lung vascular permeability in two different models of ALI: ischemia-reperfusion in rats (mediated by vascular endothelial growth factor [VEGF]) and ventilation-induced lung injury (VILI) in mice (mediated, at least in part, by transforming growth factor-beta [TGF-beta]). Knockout mice homozygous for a null mutation of the integrin beta5 subunit were also protected from lung vascular permeability in VILI. In pulmonary endothelial cells, both the genetic absence and blocking of alphavbeta5 prevented increases in monolayer permeability induced by VEGF, TGF-beta, and thrombin. Furthermore, actin stress fiber formation induced by each of these agonists was attenuated by blocking alphavbeta5, suggesting that alphavbeta5 regulates induced pulmonary endothelial permeability by facilitating interactions with the actin cytoskeleton. These results identify integrin alphavbeta5 as a central regulator of increased pulmonary vascular permeability and a potentially attractive therapeutic target in ALI.

Endothelial signaling during development

Blood vessels perfuse all tissues in the body and mediate vital metabolic exchange between tissues and blood. Increasing evidence, however, points to a direct role for paracrine signaling between blood vessel cells and surrounding target organ cells, during embryonic development and cell differentiation. Understanding the nature of this signaling and its heterogeneity, both in the embryo and in adult tissues, may not only provide insights into mechanisms for normal developmental cell fate decisions, but could also lead to novel targeted therapeutic approaches for a variety of diseases such as heart disease, diabetes or cancer.

Probing the structural and molecular diversity of tumor vasculature

The molecular diversity of the vasculature provides a rational basis for developing targeted diagnostics and therapeutics for cancer. Targeted imaging agents would offer better localization of primary tumors and metastases, and targeted therapies would improve efficacy and reduce side effects. The development of targeted pharmaceuticals requires the identification of specific ligand-receptor pairs, and knowledge of their cellular distribution and accessibility. Using in vivo phage display, a technique by which we can identify organ-specific and disease-specific proteins expressed on the endothelial surface, it is now possible to decipher the molecular signature of blood vessels in normal and diseased tissues. These studies have already led to the identification of peptides that target the normal vasculature of the brain, kidney, pancreas, lung and skin, as well as the abnormal vasculature of tumors, arthritis and atherosclerosis. Membrane dipeptidase in the lungs, interleukin-11 receptor in the prostate, and aminopeptidase N in tumors are examples of molecular targets on blood vessels. Corresponding confocal-microscopic imaging and ultrastructural studies are providing a more complete understanding of the cellular abnormalities of tumor blood vessels, and the distribution and accessibility of potential targets. The combined approach offers a strategy for creating a ligand-receptor map of the human vasculature, and forms a foundation for the development and application of targeted therapies in cancer and other diseases.

Morphology of capillary-like structures in a three-dimensional aorta/collagen gel culture

The morphology of capillary-like tubes was investigated by electron microscopy (TEM and SEM) using an in vitro model of capillarogenesis (aorta/collagen type I gel). This model allowed morphological comparisons with in vivo capillaries and an evaluation of the functional maturity of the endothelium to be made. The lumina developing in vitro were demarcated by endothelial cells of varying thickness (0.1-2 microns). Pericytes were resting on the outside. The endothelial cells were characterized by contacts of varying length with tight and gap junctions and occasional indentations. The inner surface exhibited areas both with pronounced and without any endocytotic activity. In addition to a large Golgi apparatus, a varying number of cell organelles occurred depending on the thickness of the endothelium. Bundles consisting of microfilaments were often located underneath the outer cell membrane and in the vicinity of contact areas. A lamina densa was in the process of formation. The capillaries grown in vitro closely resembled those in vivo and showed a high degree of differentiation. Hence, this in vitro model allows the study of a number of functions of endothelial cells.

Heterogeneous histochemical reaction pattern of the lectin Bandeiraea (Griffonia) simplicifolia with blood vessels of human full-term placenta

Bandeiraea simplicifolia lectin (BS-I) stains vascular endothelium in various species. In humans, less than 10% of the specimens studied exhibit a reaction with BS-I. In the present histochemical study, the reactivity of BS-I with placental blood vessels and its correlation with the blood group from mother and newborn child was investigated. Acetone-fixed cryosections of representative tissue segments of human full-term placenta and umbilical cord were stained with BS-I. The staining pattern of tissues from patients with different blood groups was identical, although the reaction of BS-I in the placenta was heterogeneous. BS-I did not react with the umbilical cord. Vascular smooth muscle cells at the insertion site of the umbilical cord into the chorionic plate, and endothelium deeper in the chorionic plate, became progressively stained. The endothelial cells and tunica muscularis of smaller arteries and veins in stem villi lost their reactivity in parallel with decreasing vessel size. Arterioles and venules reacted heterogeneously. Capillaries, trophoblastic basement membranes, especially epithelial plates, and sometimes the syncytiotrophoblast were labelled in several terminal villi. The data indicate that 1) the placenta binds BS-I to fetal endothelium independent of the blood group, 2) cell-surface antigens on placental endothelial cells are expressed heterogeneously and 3) cell-surface glycans are constituted in an organ-specific manner on human endothelial cells.

A new reliable method of culturing and measuring human decidual capillary endothelial cells

A reliable method of culturing human decidual capillary endothelial cells was developed. Endothelial growth cell supplement, heparin and newborn bovine serum were added to the culture medium to facilitate growth. Decidual capillary endothelial cell cultures showed similar growth curves to the more traditional human umbilical cord vein endothelial cell cultures. Culture growth was assessed by a new morphometric method of measuring cell culture area with the use of a computerised measuring tablet. This new method was compared with the more laborious method of counting individual cells and was found to be as reliable and more efficient.

Immunohistochemical evidence for the heterogeneity of maternal and fetal vascular endothelial cells in human full-term placenta

The heterogeneity of endothelial cell surface antigen expression was studied in 5 human full-term placentae by means of indirect immunohistochemistry using 9 monoclonal antibodies and by staining with fluorescent-conjugated Ulex europaeus lectin, both of which are widely used endothelial cell markers. (1) A highly specific, homogeneous staining of fetal and maternal placental vessels of all sizes and anatomical regions was observed by the monoclonal antibodies PAL-E, QBEND10 and 1F10. These antibodies were even more specific than Ulex europaeus lectin, factor VIII antibody and von Willebrand factor antibody, which cross-reacted with some non-endothelial cells and structures. The reactivity of PAL-E, QBEND10 and 1F10 with residual surface cells of the basal plate strongly suggests an endothelial origin of these cells. (2) In contrast to other organs, PAL-E, QBEND10 and HM15/3 strongly stained endothelial cells of the macrovascular system in the human placenta. This might indicate an organ-associated heterogeneity of fetal endothelial cells. (3) Monoclonal antibodies against receptors for transferrin and IgG (Fc gamma RII) labeled the endothelial cells of fetal placental vessels with increasing intensity distal to the insertion of the umbilical cord. The vessels of the umbilical cord itself were unreactive. This might suggest a heterogeneity of macro- and microvascular endothelial cells.

Vascular permeability changes in tumours of the peripheral nervous system

Vascular permeability changes were examined in 34 tumours of the peripheral nervous system by immunohistochemical demonstration of serum proteins as endogenous tracers. The blood-tumour barrier was impaired in the reticular (Antoni type B) portions of neurinomas (Schwannomas) and in cutaneous neurofibromas but was similar to the normal blood-nerve barrier in fibrillary (Antoni type A) neurinomas, in most neurofibromas, in ganglioneuromas and in anaplastic tumours. These differences in permeability are discussed in relation to aspects of pathological tumour vascularization, the histogenesis of microcystic changes, and systemic therapeutic approaches.

Comparison of oxidative metabolism in vitro in endothelial cells from different species and vessels

Oxygen consumption was compared in confluent cultures of endothelial cells from human umbilical cord veins, rat pulmonary arteries, and bovine aortas. A microrespirometric method utilizing oxygenated hemoglobin as oxygen supply and indicator of respiration was used. Respiratory rate was equal in human and murine cells (2.0 X 10(-6) and 2.2 X 10(-6) microliters O2/cell/hour respectively), compared on a cell-to-cell basis, while respiratory rate of the bovine endothelium was significantly lower (0.4 X 10(-6) microliters O2/cell/hour) (P less than .001).

Capillary permeability in the pancreas and colon: restriction of exogenous and endogenous molecules by fenestrated endothelia

The permeability properties of fenestrated capillaries in the colon and exocrine and endocrine pancreas to exogenous and endogenous molecules were examined. The exogenous tracers horseradish peroxidase (Einstein-Stokes radius [ESR], 3.0 nm), hemoglobin (ESR, 3.2 nm), and ferritin (ESR, 6.1 nm) were injected intravenously and allowed to circulate for 5-90 min. Tissues were removed and processed for cytochemical or standard electron microscopic examination. The endogenous plasma proteins albumin (ESR, 3.5 nm) and IgG (ESR, 5.5 nm) were localized by immunocytochemistry using the protein A-gold technique. All vessels examined were permeable to HRP in less than 5 min. In contrast, these vessels were restrictive to the slightly larger hemoglobin molecule (60-min circulation) and to ferritin (90-min circulation). Capillaries in the exocrine and endocrine pancreas were restrictive to albumin and IgG. These results demonstrate the presence of fenestrated capillary beds, in addition to the choriocapillaris, that are restrictive to molecules with ESR greater than or equal to 3.2 nm. Capillaries in the mucosa of the colon were restrictive to hemoglobin and ferritin but did not restrict albumin or IgG. This indicates that these vessels are of the permeable type. However, the rate of transendothelial movement of molecules is slower than that of other permeable vessels, such as in the ileo-jejunum. This study has provided further evidence for the existence of fenestrated endothelia that are restrictive to exogenous and/or endogenous molecules.

Cluster microvilli in coronary endothelium

Examination of cardiac vascular endothelium with scanning electron microscopy, and transmission electron microscopy of previously scanned tissue, revealed several regions of the coronary venous system that contained 'cluster' microvilli. These consisted of 2-15 microvillous projections that emanated radially from a common base or were grouped into a fan-like arrangement. Although rare, these clusters, when present, were widely distributed over the endothelial cell surface.

Microvasculature of human colorectal epithelial tumors. An electron microscopic study

The ultrastructure of the microvasculature in human colorectal adenomas and carcinomas was studied, and compared with that of normal tissue. Blood vessels in adenomas were generally of normal structure, whereas carcinoma vessels displayed a variety of structural alterations. Endothelial proliferation was frequently observed in all the eleven carcinomas examined, which confirms the reports of new vessel formation in experimental malignant tumors. The presence of fenestrations in obviously abnormal endothelium was tentatively attributed to hypoxia, vascular regression or immaturity of the cells, in spite of the fact that normal capillaries in colonic mucosa are fenestrated. Some vessels showed multilayered basement membranes, various types of activated cells containing numerous lysosomal granules, and thickened perivascular tissue. These features may be a vascular response to repeated damage or a manifestation of vascular remodelling. Dilated thin-walled vessels, which were usually found at the periphery of carcinomas, were identified as venules. Small vessels located between the closely apposed carcinoma tubules were not obviously abnormal. They were nevertheless considered to be tumor-induced vessels which had undergone differentiation. The differences in vascular morphology between adenomas and carcinomas were considered to be due to their different growth patterns, growth rates and degrees of maturity.

Vascular changes in psoriatic synovium. A light and electron microscopic study

Light and electron microscopic studies were performed on the synovial membranes from the large joints of 12 patients with psoriatic arthritis. The most significant histopathologic findings were vascular changes that were characterized by endothelial cell swelling accompanied by inflammatory cell infiltration and marked thickening of the vessel wall. Fibrosis of the subsynoviocyte tissue was also prominent, especially in patients with chronic disease. Villi were not prominent, and the lining layer was not thickened. Both type A and B cells were seen; none had significant abnormalities. No electrondense deposits were observed. These findings lend further support to the theory that microvascular changes play a significant role in the pathogenesis of psoriasis and its related arthritis.

Histiocytoid endothelial cells in a malignant schwannoma

Unusual vascular lining cells within a malignant schwannoma prompted a comparative study of these cells and the endothelial cells in cases of histiocytoid hemangiomas. The endothelial cells in all cases showed similar morphologic and immunohistochemical features. Such endothelial cells were not found in the capillaries of a pyogenic granuloma or in those of developing rat skin. It is postulated that these distinctive endothelial cells may be more widely distributed than was previously thought and that morphologic changes associated with these cells may be induced by vasoactive substances.

Fetal and neonatal rat intestinal capillaries: a TEM study of changes in the mural structure

The development of intestinal mucosal capillaries was studied in Wistar rats from the fourteenth day of gestation through the second postnatal day with the transmission electron microscope. The microvessels develop first as continuous capillaries without a basal lamina and become fenestrated during later stages of gestation. In the thickened areas of the cytoplasm of endothelial cells at early stages, there is a paucity of pinocytotic vesicles; but some vesicles are observed which are larger than those in the adult. Later in gestation these larger vesicles can still be seen, but the frequency of their occurrence decreases with the maturation of the vessels. The intercellular junctions vary in length and shape, and the space between the outer leaflets of the apposed cell membranes is usually 20 nm. Surrounding the abluminal surface of the endothelium, the incipient basal lamina appears attenuated and incomplete until at least the second day of neonatal life.

The development of the sinusoids of fetal rat liver: morphology of endothelial cells, Kupffer cells, and the transmural migration of blood cells into the sinusoids

The fine structural development of rat fetal liver sinusoids from 10 to 22 days gestation was studied. Colloidal carbon (Pelikan ink) was injected into 14-22 day gestation fetuses via the umbilical vein to assess the continuity of the sinusoidal lining and the phagocytic ability of the developing lining cells. Endothelial cells, devoid of an underlying basal lamina, form the bulk of the vascular lining at all gestational ages. These cells possess typical intercellular junctions and fenestrae with diaphragms before 17 days gestation. Transendothelial open fenestrations, typical of the adult liver, appear around 17 days gestation, increasing in number for the remainder of gestation. Although fenestrae possessing diaphragms are permeable to carbon before 16 days gestation, open fenestrations, first seen at 17 days gestation, allowed large amounts of carbon to reach the extravascular space. Endocytosis of carbon by endothelial cells was accomplished exclusively by large bristle-coated vesicles. Endothelial cells were also seen to be involved in transmural diapedesis of newly formed erythrocytes and megakaryocyte processes from the extravascular space by forming a temporary migration pore allowing these cells and processes to enter the circulation. At the end of gestation, blood-forming activity had nearly ceased, and only the space of Dissé separated the lining cells from the parenchymal cells. Kupffer cells were easily identified as early as 13 days gestation by their content of phagosomes and engulfed erythrocytes. The Kupffer cells are much more avid in the phagocytosis of carbon than are endothelial cells. Toward the end of gestation, some Kupffer cells develop a homogeneous "sticky coat" to carbon.

Fetal cardiovascular system as influenced by maternal smoking

Primary prevention of atherosclerosis should be initiated early in life, preferably in childhood. Nevertheless, it tends to be forgotten that a child has existed for nine months before birth, and that the newborn child might already have been exposed to agents capable of causing vessel damage. Studies of the umbilical artery, umbilical vein, and vessels of the placental villi (2, 3, 4, 5, 6) revealed that, in this portion of the fetal cardiovascular system, severe damage to the vessel wall is associated with maternal tobacco smoking during pregnancy. These alterations within the fetal cardiovascular system were never found in the children of non-smoking mothers. If similar changes occur in the other vessels of the newborn child, which it seems reasonable to expect, they might give rise to sequelae later in life. It should therefore strongly be advocated that pregnant women abandon tobacco smoking.